The present invention relates to an optical element having a spherical, aspherical, or free surface, and a mold and mold structure for molding the optical element.
In particular, the present invention relates to a mold and mold structure used when manufacturing an optical element, in which optical functional surfaces as free surfaces are continuously arranged, by molding a resin material.
The present invention also relates to an optical element in which a mold member for forming a cavity surface that molds a curved spherical, aspherical, or free surface is constituted by a plurality of optical inserts.
In particular, the present invention relates to an optical element in which a plurality of curved optical functional surfaces are continuously arranged.
In the technique of molding a camera lens and a prism having a curved optical functional surface, e.g., an aspherical, spherical, or free surface, a toric lens formed of a generatrix curved surface and a directrix curved surface that have different radii of curvature, or the like by injecting a resin material into a mold and cooling and solidifying the resin material, to mold an optical surface shape precisely as designed requires a very high-level machining/assembling technique in formation of the cavity surface of a mold that forms the optical surface.
In particular, when an optical element having light incident and exit surfaces each of which is not composed of a single optical surface but of a combination of a plurality of curved optical surfaces is to be molded by injecting a resin material into a mold and cooling and solidifying the resin material, to mold the optical element such that its plurality of free surfaces satisfy a relative positional relationship precisely as designed requires a very high technical level.
For example, the optical element shown in FIG. 1 is a lens used in, e.g., a video camera, a still video camera, a finder optical system, an image input apparatus, an image forming apparatus, or the like. In this optical element, a plurality of optical surfaces form one lens surface, and optical functional surfaces are adjacent to each other. The respective optical surfaces are designed to maintain a certain positional relationship with each other.
If the lens optical surface has a complicated shape, a technique that molds this lens by placing a nest in a mold is known (Japanese Patent Laid-Open No. 5-96580).
Japanese Patent Laid-Open No. 4-65210 proposes a following mold. A recess is formed in either one of stationary and movable cavity blocks, and a projection is formed in the remaining one of the stationary and movable cavity blocks. The stationary and movable molds are then aligned. This aims at eliminating misalignment between the optical axis of the aspherical surface of a lens incident surface and the optical axis of the spherical surface of a lens exit surface, as in an aspherical lens.
When the transfer surface that constitutes a cavity for molding a free surface shape is composed of a free surface, and particularly when a plurality of free surfaces must be arranged side by side continuously, this transfer surface (cavity surface) is divided into a plurality of specular optical inserts.
To suppress eccentricity or misalignment of the specular optical inserts within an allowable range, the clearance between the specular optical inserts and the cavity block pocket which is to accommodate the specular optical inserts must be minimized. Partly due to the limit of the machining precision of the components, if the error in size of the specular optical insert or of the specular optical insert pocket of the cavity block is large, the gap between the specular optical inserts and the cavity block causes backlash to produce eccentricity and misalignment, largely affecting the dimensional precision of the respective aspherical surfaces.
Depending on the size error, the specular optical inserts may be press-fitted into the specular cavity block pocket. This causes distortion in the cavity-forming surfaces of the specular optical insert to adversely affect the optical performance of the optical element to be molded.
FIG. 1 shows an optical element 1 having an optical surface formed by arranging a plurality of optical functional surfaces 1A, 1B, 1C, . . . as free surfaces side by side. If this optical element 1 is to be injection-molded by using a resin material, the cavity surfaces of a mold member, in which the resin material is to be injected, cooled, and solidified to mold the optical element 1, must have a predetermined optical positional relationship with each other. Also, in order to form the cavity surfaces, the positional relationship among the respective mold portions must be adjusted.
An optical prism having curved reflecting surfaces is disclosed in, e.g., U.S. Pat. No. 4,775,217 and Japanese Patent Laid-Open No. 2-297516.
In the optical prism having the curved reflecting surfaces, since degradation in optical performance due to eccentricity of the reflecting surfaces is generally larger than in an optical prism constituted of only flat surfaces, the positional precision of each reflecting surface is very strict.
The technique shown in U.S. Pat. No. 4,775,217 and Japanese Patent Laid-Open No. 2-297516 does not refer to a reflecting surface adjusting method, assembling method, and manufacturing method that guarantee the positional precision of each reflecting surface.
When the number of reflecting surfaces of the optical prism is increased, eccentricities of the respective reflecting surfaces are accumulated. The larger the number of reflecting surfaces, the smaller and stricter the amount of eccentricity allowable for each reflecting surface. Hence, a technique that guarantees the positional precision of each reflecting surface is sought for.
Conventionally, as a method of incorporating a nested piece for suppressing eccentricity, the following two methods are known. According to the first method, the difference between the inner diameter of a bag-like piece holder and the outer size of a nested piece is minimized to suppress the fitting gap. According to the second method, as described in Japanese Patent Laid-Open No. 5-9680, the outer walls of optical inserts are urged against the inner walls of mold base by optical insert press plates with screw shafts. The optical inserts are to be incorporated inside the mold bases. The optical insert press plates oppose the outer walls of the optical inserts and are formed at the distal ends of screw shafts. The screw shafts generate axial forces perpendicular to the inner wall surfaces of the templates perpendicular to each other.
These conventional methods have the following drawbacks. Even if the clearance between the optical insert and the cavity block for incorporating it is minimized to suppress the eccentricity or misalignment of the optical insert within an allowable range, when incorporating the optical insert, since this clearance is always required to incorporate the optical insert in the cavity block, the eccentricity corresponding to the amount of this clearance cannot be removed. When the outer size of the optical insert or the inner diameter of the cavity block should have an error, if small, then backlash is produced to cause eccentricity or misalignment, or the nested piece may be press-fitted in the piece holder to generate distortion on the cavity-constituting surface of the optical insert. This results in parallel eccentricity and tilt eccentricity.
As the requirements for the parallel eccentricity and tilt eccentricity become strict, a right angle between the inner surface of the cavity block and the bottom surface of the optical insert, a right angle between the side surfaces of the cavity block and optical insert, flatness of the side surfaces of the cavity block and optical insert, and the like must be set strict, and the clearance between the cavity block and the optical insert must be suppressed. The smaller the clearance, the more difficult to incorporate the optical insert. Each time the optical insert is incorporated, its side surface and the inner surface of a cavity block pocket come into contact with each other to increase their surface roughness. Then, the contact state becomes unstable, and the flatness and right angles become inaccurate.
A method of urging an optical insert against the inner surface of a cavity block from two orthogonal side surfaces through a block can be applied to a large-scale structure in which one optical insert is used and, if the optical insert has a size exceeding a certain degree, is adjusted with a screw shaft. This method cannot be applied to a plurality of optical surfaces, which must be arranged side by side in one optical component, due to a structural impossibility. When the plurality of optical inserts are used, depending on the contact state among the contact surfaces of the optical inserts, a tilt or rotational deformation occurs in each nested piece. Therefore, the eccentricity of each optical surface with respect to the optical axis must be controlled.